A standard split-jaw lathe chuck has a chuck body rotatable about a chuck axis and formed with a plurality of angularly spaced and radially extending inner guides receiving respective inner jaw parts, and with a plurality of angularly spaced and radially extending outer guides receiving respective outer jaw parts that project axially from the chuck body. Respective coupling members are axially displaceable in the inner jaw parts between axially advanced coupling positions radially linking the respective jaw parts and axially retracted decoupling positions permitting relative radial displacement of the respective jaw parts. These coupling members may be jointly displaceable as described in U.S. Pat. No. 3,682,491 or independently displaceable as described in my earlier copending application Ser. No. 886,348 filed Mar. 14, 1978.
The advantage of such a structure is that it allows the outer jaw parts to be roughly adjusted for the workpiece to be clamped, so that only a modest radial stroke is necessary for the jaws to clamp the workpiece. As a result a large mechanical advantage can be gained in the device so that the operating member moves through a relatively long axial stroke to produce a relatively short radial stroke of the jaws. Hence a good mechanical advantage is obtained for extremely tight gripping of a workpiece. What is more exchanging of the jaws, or using jaw blanks such as described in my copending application Ser. No. 40,126 filed May 18, 1979, is also possible.
One of the recurring problems with such a chuck is that when spun during the machining operation the radially outwardly effective centrifugal forces on the jaws loosen the hold of these jaws on the workpiece. Thus it is known to provide force compensation such as described in my copending application Ser. No. 40,271 filed May 18, 1979, or in U.S. Pat. No. 2,729,459. In such an arrangement radially displaceable weights on the chuck body engage via force-converting members with the inner jaw parts so that radially outwardly effective centrifugal forces in these weights are converted into radially inwardly effective forces applied to the inner jaw parts. In this manner centrifugal forces can be balanced in the chuck.
These centrifugal forces are advantageously emloyed in split-jaw lathe chucks of the type described in my above-cited copending application Ser. No. 886,348 wherein radially displaceable cams are provided in each of the inner jaw parts engaging the respective coupling members. These cams are radially displaceable between outer positions urging the coupling members forwardly into the coupling positions and radial inner positions allowing the coupling members to move axially backwardly. Thus in such a system the radially outwardly effective forces on these cams ensure that as rotation speed increases the forces pressing the coupling members into engagement with the respective outer jaw parts also increase.
My above-cited copending application Ser. No. 40,128 describes a chuck of the above-described general type and provided with a release ring rotatable on the chuck about its axis and formed with at least one radially inwardly extending projection engageable with the cams to move same into the inner positions against the forces of the respective springs that urge them outwardly. Otherwise these cams are received within inwardly open recesses of the rings and are completely covered thereby so that rotation of this release ring can decouple any of the inner and outer jaw parts.
Such arrangements, and others such as seen in my copending application Ser. No. 40,272 filed May 18, 1979, represent a substantial improvement over the prior art. Nonetheless such devices occasionally have shown some problems, in particular when force compensation is used in a split-jaw chuck.